1. Field of the Invention
The present invention relates to an optical transmitter for use with an optical fiber communication system.
Recently, the so-called DPSH-IM (direct phase-shift and self-homodyne intensity modulation) has been proposed as a modulation method which is highly immune to the effect of wavelength dispersion and which dissipates low levels of modulation driving power compared with other methods. The DPSH-IM works as follows: Varying the injection current of a laser diode first modulates the oscillated light waves therefrom in phase. The phase-modulated light is then converted into intensity-modulated light by a self-homodyne arrangement. With the DPSH-IM, a Mach-Zehnder interferometer provides the self-homodyne capability. This requires stabilizing the operating point involved in order to prevent waveform distortion. Furthermore, to address the fluctuation in the frequency modulation efficiency of the laser diode requires controlling the driving amplitude involved.
Meanwhile, optical transmitters for coherent light wave communication, to which the so-called CP-FSK is applied, also require control over the driving amplitude. Optical transmitters in which the light from a constantly driven laser diode is intensity-modulated by a Mach-Zehnder optical modulator require control over the operating point and driving amplitude.
2. Description of the Related Art
Under the DPSH-IM method, a laser diode fed with a bias current larger than a threshold current is supplied with modulation current pulses of a small amplitude for phase-modulating the oscillated light. The phase-modulated light is converted by passage through an optical interferometer into intensity-modulated light. The operating principle of the DPSH-IM is described in more detail in "Fibre Transmission Properties of Optical Pulses Produced Through Direct Phase Modulation of DFB Laser Diode" by Shirasaki M., Nishimoto H., Okiyama T and Touge T (ELECTRONICS LETTERS, 14th Apr., 1988, Vol. 24, No. 8, pp. 486-488). One PCT application related to the DPSH-IM is PCT/JP89/00220.
The DPSH-I permits small amplitude modulation of the laser diode under large bias currents. This makes it possible to construct a system highly immune to the adverse effects of chirping and operating on low driving voltages.
In a DPSH-IM setup, slight changes in the oscillation frequency of the laser diode or in the delay time difference of the optical interferometer cause the optical signal from an optical interferometer to be distorted or inverted in polarity. The oscillation frequency of the laser diode and the delay time difference of the optical interferometer are known to vary depending on temperature and aging characteristics. Thus where an optical transmitter based on the DPSH-IM is used for practical purposes, it is necessary to control the oscillation frequency of the laser diode or the delay time difference of the optical interferometer in order to stabilize the operating point. Stabilization of the operating point is needed to prevent waveform distortion and polarity inversion in signals. A simple prior art description about how to stabilize the operating point is found in "Field Demonstration of FSK Transmission at 2.488 Gigabits/second over a 132 km Submarine Cable Using an Erbium Power Amplifier" by E. G. Bryant et al. (Topical Meeting on Optical Amplifiers and Their Applications, 1990, pp. 152-155). According to the above publication, the operating point is stabilized apparently by having the bias current of a laser diode modulated with a small signal of 10 kHz and by having the output light from an optical interferometer monitored in order to control the bias current.
The rate of fluctuation in the oscillation frequency (FM efficiency) with respect to the amplitude of the current for driving the laser diode is predictably varied depending on the temperature and aging characteristics of the laser diode. Thus, in order to prevent waveform distortion, it is necessary to ensure continuous control of the oscillation frequency based on the driving currents of an appropriate amplitude.